1. Field of the Invention
The present invention relates to a spherical, single-crystal ferrite powder which has excellent magnetic properties.
2. Description of the Related Art
Spinel or magnetoplumbite ferrite represented by the general formula MFe2O4 or MFe12O19 (where M is one or more bivalent metal ions), as well as ferrite represented by Mxe2x80x23Fe5O12 (where Mxe2x80x2 is a trivalent metal ion) and lithium ferrite represented by Li0.5Fe2.5O4 are used in a variety of applications as magnetic materials having excellent magnetic properties.
For example, soft ferrites such as Fe3O4, NiFe2O4, MnFe2O4, (Ni,Zn)Fe2O4, (Mn,Zn)Fe2O4 and CoFe2O4 are used as soft magnetic materials where high magnetic permeability, low coercive force and low loss are required, as in cores of inductors, transformers, and filters; magnetic head cores; magnetic shielding materials; etc., and they are also used as magnetostrictive materials. xcex3-Fe2O3 and barium ferrite, etc., which have great crystal magnetic anisotropy, are used as permanent magnet materials and high-density magnetic recording materials.
Ferrite powders having a variety of properties are used as the raw materials for these applications, but a truly spherical and fine single-crystal ferrite powder with excellent magnetic properties within the desired range of particle sizes has not been known until now.
For example, the most widely used ferrite powders are manufactured by the solid phase reaction (sintering) method wherein oxides or carbonates of constituent metals are mixed and subjected to prolonged heat treatment at temperatures in excess of one thousand and several hundreds degrees C., and the resulting composite oxides are pulverized. However, ferrite powders obtained by this method are highly aggregated polycrystalline powders in irregular shape. Although it is possible to pulverize them finely until they are substantially single-crystal powder, they are then angular and nonuniform in shape. For example, while the manufacture by sintering of a magnetoplumbite ferrite single-crystal powder is described in Japanese Patent Application Laid-open No. 9-48618 (U.S. Pat. No. 5,736,111), it is an angular, polyhedral powder with a variety of shapes.
The spray roasting methods described in Japanese Patent Publication No. 47-11550 and 63-17776 and Tatsushiro Ochiai""s xe2x80x9cDevelopment of Ferrite Raw Powder Production Process with Thermal Decomposition of Iron and Manganese Chloride solution by Spray Roasterxe2x80x9d (Journal of the Japan Society of Powder and Powder Metallurgy Vol. 45, No. 7, p. 624) produce fine ferrite powders by spraying and thermal decomposition of solutions or suspensions of the raw materials, which are chlorides, oxides, nitrates, etc. of the component metals. Ordinarily this thermal decomposition reaction takes place at temperatures below 1000xc2x0 C., and the resulting ferrite powder is a polycrystalline powder in polyhedral or nonuniform shape. Depending on the conditions, it may become spherical, but has a low crystallinity.
Neither are there any reports of manufacturing spherical and single-crystal fine powder by wet methods such as hydrothermal synthesis or by vapor phase reaction. In particular, while it is possible to manufacture ultrafine powder by the vapor phase reaction method, spherical and single-crystal fine powder with a mean particle size of about 0.1 to 30 xcexcm is not known to have been obtained.
Moreover, Japanese Patent Application Laid-open 9-169523 describes a ferrite fine powder with a mean particle size of 0.1 xcexcm or more manufactured by vapor phase epitaxy, wherein the ratio of the saturation magnetization value of the powder to the saturation magnetization value of the fired product is 70% or more. After being molded by a variety of molding methods, this ferrite fine powder is further fired to achieve the final magnetic properties and used, or is used as an intermediate raw material. This fine powder is manufactured by spraying of an aqueous metal chloride solution in a combustion flame, evaporation of the metal chloride as the raw material in a process of thermal decomposition at temperatures not higher than 1000xc2x0 C., and vapor phase epitaxy of spinel single crystals around nuclei of iron oxide deposited in the vapor phase; as described, it involves complex process control resulting in fine particles with good magnetic properties and a mean particle size of about 0.1 xcexcm. As with the vapor phase reaction method mentioned above, however, it is difficult with this method to grow large particles while maintaining their spherical shape and the state of single-crystal particles, and it is not possible to obtain spherical, single-crystal fine powder with a mean particle size of 0.1 to 30 xcexcm, or in particular 0.3 to 30 xcexcm.
There is also the technology of melting ferrite, making bulk single-crystal product from the liquid phase and producing single-crystal spheres through grinding it into a spherical shape, but such spheres have limited use because they are large, generally 0.5 mm or more.
Control of the physical properties of the ferrite powder which is the raw material is vital in the manufacture of magnetic bodies. For example, the soft ferrite used in cores of coils and transformers needs to have a low coercive force (Hc) in order to minimize drive magnetic fields, as well as a low hysteresis and good linearity in its magnetization curve. In recent years, moreover, improvements in the electrical and magnetic properties of magnetic materials, such as higher magnetic permeability, decreased loss and improved frequency characteristics, are required in order to produce better inductance and high-frequency characteristics. Therefore, it is extremely important that the magnetic properties of ferrite powder by improved, along with physical properties such as shape, particle size and reactivity. Specifically, there is a demand for spherical fine powders which have a single-crystal structure relatively unaffected by grain boundaries and impurities, which do not aggregate, and which are highly dispersible and packable.
Specifically, when manufacturing sintered cores and permanent magnets by molding and sintering processes, if the raw material is a polycrystalline powder with a nonuniform shape, localized abnormal crystal growth and heterogeneous composition tend to occur, and it is impossible to obtain a dense, high-performance ferrite sintered body with excellent magnetic properties and mechanical strength. On the other hand, when ferrite powder is compacted with polymer materials such as resin and gum in the manufacture of dust cores, for example, in order to obtain a final product with good magnetic properties without molding followed by sintering, it is important that the powder itself have excellent magnetic properties, that it can be dispersed uniformly in order to minimize variation in its properties, and that the packing density can be enhanced. To this end, a spherical, single-crystal powder with a mean particle size of 0.1 to 30 xcexcm, and particularly 0.3 to 30 xcexcm, which does not aggregate, which is fine and uniform in shape and particle size and which has low surface activity would seem to be ideal.
In the past, however, no powder fulfilled all of these requirements, so there were limits to improving the magnetic properties.
With the foregoing in view, it is an object of the present invention to provide a novel ferrite fine powder having superior powder physical properties and magnetic properties not found in existing magnetic materials. In particular, a ferrite fine powder is provided which has superior properties as a raw material for ferrite sintered bodies or a material for a dust core, which has high dispersibility and packablity, and which also has magnetic properties suitable for high-frequency applications.
The present invention provides a ferrite fine powder which consists of spherical single crystal particles having a mean particle size of 0.1 to 30 xcexcm. Moreover, the present invention provides a spherical, single-crystal ferrite fine powder with a mean particle size of 0.1 to 30 xcexcm and a sphericity in the range of 0.95 to 1. Still further, the present invention provides a spherical, single-crystal ferrite fine powder with a mean particle size of 0.1 to 30 xcexcm manufactured by spray pyrolysis.